Divided-aperature infra-red spectral imaging system for chemical detection

ABSTRACT

A divided-aperture infrared spectral imaging (DAISI) system that is structured to provide identification of target chemical content in a single imaging shot based on spectrally-multiplexed operation. The system is devoid of spectral scanning acquisition of infrared (IR) spectral signatures of target content with an IR detector and does not require content.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/543,692, filed on Nov. 17, 2014, which is a continuation ofInternational Application No. PCT/US2013/041278, filed on May 16, 2013which claims benefit of and priority from the U.S. ProvisionalApplications Nos. 61/688,630 filed on May 18, 2012 and titled “DividedAperture Infrared Spectral Imager (DAISI) for Chemical Detection”, and61/764,776 filed on Feb. 14, 2013 and titled “Divided Aperture InfraredSpectral Imager for Chemical Detection”. The disclosure of each of theabove-mentioned applications is incorporated by reference herein in itsentirety.

TECHNICAL FIELD

The present invention generally relates to a system and method for gascloud detection and, in particular, to a system and method of detectionof spectral signatures of chemical compositions in a mid- and long-waveinfrared spectral region with a use of systemic compensation forparallax-induced and motion-induced imaging artifacts.

BACKGROUND

Most of the existing IR spectral imaging systems require focal planedetector arrays (FPAs) that have to be highly sensitive and cooled inorder to compensate, during the optical detection, for the reduction ofthe photon flux caused by spectrum-scanning operation. There remains aneed, therefore, in a system enabling an optical data acquisition modethat does not require the cooling of the used detector(s), whichdetectors can be less sensitive to photons in the IR but yet well fitfor continuous monitoring applications. There also remains a need in anIR imaging system the operation of which is substantially notsusceptible to motion artifacts (which is a common problem withspectrally-scanning systems causing errors in either the spectral data,spatial data, or both).

SUMMARY

Embodiments of the present invention provide an infrared (IR) imagingsystem for determining a concentration of a target species in an object.The imaging system includes (i) an optical system, having an opticalfocal plane array (FPA) unit that is devoid of a cooling means, whichoptical system is configured to receive IR radiation from the objectalong at least two optical channels defined by components of the opticalsystem, said at least two optical channels being spatially andspectrally different from one another; (ii) first and secondtemperature-controlled shutters removably positioned to block IRradiation incident onto the optical system from the object; and (iii) aprocessor configured to acquire multispectral optical data representingsaid target species from the received IR radiation in a singleoccurrence of data acquisition. The optical system may include anoptical aperture (a boundary of which is defined to circumscribe,encompass said at least two spatially distinct optical channels) and atleast two spectrally-multiplexed optical filters. Each of these opticalfilters is positioned to transmit a portion of the IR radiation receivedin a respectively corresponding optical channel from the at least twospatially and spectrally different optical channels and includes atleast one of a longpass optical filter and a shortpass optical filter(with or without a combination with another filter such as a notchfilter, for example). The optical system may further include at leasttwo reimaging lenses, each reimaging lens disposed to transmit IRradiation (in one embodiment—between about 1 micron and about 20microns), that has been transmitted through a corresponding opticalfilter towards the optical FPA unit. In one embodiment, the optical FPAunit is positioned to receive IR radiation from the object through theat least two reimaging lenses to form respectively-corresponding two ormore sets of imaging data representing the object and the processor isconfigured to acquire said optical data from the two or more sets ofimaging data.

Embodiments of the present invention additionally provide a method foroperating an infrared (IR) imaging system. The method includes receivingIR radiation from an object along at least two optical channels definedby components of an optical system of the IR imaging system, which atleast two optical channels are spatially and spectrally different fromone another. The method further includes transmitting the received IRradiation towards an optical focal plane array (FPA) unit that is notbeing cooled in the course of normal operation; and removablypositioning at least one of at least two temperature-controlled shuttersin front of the optical system to block IR radiation incident onto theoptical system from the object.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood by referring to thefollowing Detailed Description in conjunction with the Drawings, ofwhich:

FIG. 1 shows an embodiment of the system of the invention utilizing acommon front objective lens that has a pupil divided spectrally andre-imaged onto an infrared FPA.

FIG. 2 shows an embodiment with a divided front objective lens and anarray of infrared sensing FPAs.

FIG. 3A represents an embodiment employing an array of front objectivelenses operably matched with the re-imaging lens array.

FIG. 3B illustrates a two-dimensional array of optical componentscorresponding to the embodiment of FIG. 3A.

FIG. 4 is a diagram of the embodiment employing an array of fieldreferences and an array of respectively corresponding relay lenses.

FIG. 5A is a diagram of a 4-by-3 pupil array of circular optical filters(and IR blocking material among them) used to spectrally divide anoptical wavefront imaged with an embodiment of the invention.

FIG. 5B is a diagram of a 4-by-3 pupil array of rectangular opticalfilters (and IR blocking material among them) used to spectrally dividean optical wavefront imaged with an embodiment of the invention.

FIG. 6A depicts theoretical plots of transmission characteristics of acombination of band-pass filters used with an embodiment of theinvention.

FIG. 6B depicts theoretical plots of transmission characteristics ofspectrally multiplexed notch-pass filter combination used in anembodiment of the invention.

FIG. 6C shows theoretical plots of transmission characteristics ofspectrally multiplexed long-pass filter combination used in anembodiment of the invention.

FIG. 6D shows theoretical plots of transmission characteristics ofspectrally multiplexed short-pass filter combination used in anembodiment of the invention.

FIG. 7 is a set of video-frames illustrating operability of anembodiment of the invention used for gas detection.

FIGS. 8A, 8B are plots illustrating results of dynamic calibration of anembodiment of the invention.

DETAILED DESCRIPTION

Embodiments of the present invention illustrate a divided-apertureinfrared spectral imaging (DAISI) system that is structured and adaptedto provide identification of target chemical contents of the imagedscene based on spectrally-multiplexed operation and single-shot (alsoreferred to as snapshot), that is devoid of spectral and spatialscanning acquisition of infrared (IR) spectral signatures of the targetchemical contents with an TR detector (such as, for example, infraredfocal plane array or FPA) to form a spectral cube of imaging data. Incontradistinction to commonly used IR imaging systems, the DAISI systemdoes not require cooling.

Implementations of the present invention provide several operationaladvantages over existing IR spectral imaging systems, most if not all ofwhich require FPAs that have to be highly sensitive and cooled in orderto compensate, during the optical detection, for the reduction of thephoton flux caused by spectrum-scanning operation. The highly sensitiveand cooled FPA systems are expensive and require a great deal ofmaintenance. As an embodiment of the invention is configured to operatein single-shot acquisition mode, the instrument receives photons fromevery point of the object substantially simultaneously, during thesingle reading. In comparison with a system of related art, this featureenables an embodiment to collect a substantially greater amount ofoptical power from the imaged scene (for example, an order of magnitudemore photons) at any given moment in time. Consequently, an embodimentis enabled to operate using uncooled detector(s) (for example, FPA suchas an array of microbolometers) that are less sensitive to photons inthe IR but are well fit for continuous monitoring applications sincethey are capable of operating in extreme weather conditions, requireless power, can operate both day and night, and are less expensive. Onthe other hand, embodiments of the invention are advantageous in thattheir operation is substantially immune to motion artifacts (which is acommon problem with spectrally-scanning systems causing errors in eitherthe spectral data, spatial data, or both). Moreover, present embodimentsare structured to acquire spectrally-multiplexed datacubes during asingle-shot acquisition which, when combined with the detector-noiselimited performance of the FPA's, result in increase of level of thedetected signal by a factor of 2 to 10 times, as compared with thesystems of related art.

References throughout this specification to “one embodiment,” “anembodiment,” “a related embodiment,” or similar language mean that aparticular feature, structure, or characteristic described in connectionwith the referred to “embodiment” is included in at least one embodimentof the present invention. Thus, appearances of the phrases “in oneembodiment,” “in an embodiment,” and similar language throughout thisspecification may, but do not necessarily, all refer to the sameembodiment. It is to be understood that no portion of disclosure, takenon its own and in possible connection with a figure, is intended toprovide a complete description of all features of the invention.

In the drawings like numbers are used to represent the same or similarelements wherever possible. The depicted structural elements aregenerally not to scale, and certain components are enlarged relative tothe other components for purposes of emphasis and understanding. It isto be understood that no single drawing is intended to support acomplete description of all features of the invention. In other words, agiven drawing is generally descriptive of only some, and generally notall, features of the invention. A given drawing and an associatedportion of the disclosure containing a description referencing suchdrawing do not, generally, contain all elements of a particular view orall features that can be presented is this view, for purposes ofsimplifying the given drawing and discussion, and to direct thediscussion to particular elements that are featured in this drawing. Askilled artisan will recognize that the invention may possibly bepracticed without one or more of the specific features, elements,components, structures, details, or characteristics, or with the use ofother methods, components, materials, and so forth. Therefore, althougha particular detail of an embodiment of the invention may not benecessarily shown in each and every drawing describing such embodiment,the presence of this detail in the drawing may be implied unless thecontext of the description requires otherwise. In other instances, wellknown structures, details, materials, or operations may be not shown ina given drawing or described in detail to avoid obscuring aspects of anembodiment of the invention that are being discussed. Furthermore, thedescribed single features, structures, or characteristics of theinvention may be combined in any suitable manner in one or more furtherembodiments.

Moreover, if the schematic flow chart diagram is included, it isgenerally set forth as a logical flow-chart diagram. As such, thedepicted order and labeled steps of the logical flow are indicative ofone embodiment of the presented method. Other steps and methods may beconceived that are equivalent in function, logic, or effect to one ormore steps, or portions thereof, of the illustrated method.Additionally, the format and symbols employed are provided to explainthe logical steps of the method and are understood not to limit thescope of the method. Although various arrow types and line types may beemployed in the flow-chart diagrams, they are understood not to limitthe scope of the corresponding method. Indeed, some arrows or otherconnectors may be used to indicate only the logical flow of the method.For instance, an arrow may indicate a waiting or monitoring period ofunspecified duration between enumerated steps of the depicted method.Without loss of generality, the order in which processing steps orparticular methods occur may or may not strictly adhere to the order ofthe corresponding steps shown.

The invention as recited in claims appended to this disclosure isintended to be assessed in light of the disclosure as a whole, includingfeatures disclosed in prior art to which reference is made.

FIG. 1 provides a diagram schematically illustrating a spatial andspectral division of incoming light by an embodiment 100 of the systemof the invention (also referred to as DAISI system) that is enabled toimage an object 110 possessing IR spectral signature(s). An aperture ofthe system (associated with a front objective lens system 124) isspatially and spectrally divided. The spatial and spectral division ofthe aperture into distinct aperture portions corresponding to separatechannels 120 (in object space and/or image space) along which lightpropagates through the aperture is enabled with the use of an array 128of re-imaging lenses 128 a and an array of spectral filters 130, whichrespectively correspond to the distinct channels 120. In oneimplementation, the distinct channels 120 may include optical channelsthat are separated in angular space. The array of spectral filters 130may additionally include a filter-holding aperture mask (containing, forexample, IR light-blocking materials such as ceramic, metal, orplastic). Light from the object 110 (such as a cloud of gas, forexample), the optical properties of which in the IR are described by aunique absorption, reflection and/or emission spectrum, is received bythe aperture of the system through each of the channels 120 and isfurther imaged onto an optical detector component 136 (which may includeat least one FPA). Each of the re-imaging lenses 128 a is spatiallyaligned with a respectively-corresponding region of the divided apertureand, therefore, with respectively-corresponding spatial channel 120 toform, on the FPA component 136 a single sub-image of the object 110.Generally, two or more sub-images of the object can be characterized byclose or substantially equal spectral signatures. The FPA component 136is further operably connected with a processor 150 (not shown)specifically programmed to aggregate the data acquired with the system100 into a spectral datacube representing, in spatial (x, y) andspectral (λ) coordinates an overall spectral image of the object 110within the spectral region defined by the combination of the filters130. Additionally, the processor 150 may be optionally and specificallyprogrammed to determine the unique absorption characteristic of theobject 110 and, alternatively or in addition, map the overall imagedatacube into a cube of data representing spatial distribution ofconcentrations c of targeted chemical components within the field ofview associated with the object 110.

In order to facilitate the operational performance of the embodiment100, an optional moveable temperature-controlled reference target 160(including, for example, a shutter system containing two referenceshutters maintained at different temperatures) is removably and, in oneimplementation, periodically inserted into an optical path of lighttraversing the system 100 from the object 110 to the FPA component 130along at least one of the channels 120 to block such optical path and toprovide a reference IR spectrum required to recalibrate the operation ofthe system 100 in real time. The configuration of the moveablereference(s) 160 is further discussed below.

In the embodiment 100, the front objective lens system 124 is shown toinclude a single front objective lens positioned to establish a commonfield-of-view (FOV) for the reimaging lenses 128 a and to define anaperture stop for the whole system (which, in this specific case,substantially spatially coincides with limiting apertures correspondingto different optical channels 120). As a result, the positions forspectral encoding of the different optical channels 120 coincide withthe position of the aperture stop of the whole system, which is definedas a surface between the lens system 124 and the array 128 of thereimaging lenses 128 a. Generally, however, the field aperturescorresponding to different optical channels may be located in differentplanes. In one implementation the field apertures corresponding todifferent optical channels are located in different planes, which planesare optical conjugates of one another (as defined by the whole opticalsystem). Similarly, while all of the spectral filters 130 of theembodiment 100 are shown to lie in one plane, generally spectral filterscorresponding to different optical filters can be associated withdifferent planes. In one implementation, different spectral filters 130are situated in different planes are that are optically conjugate to oneanother.

The front objective lens element of the system can generally include anarray of front objective lenses configured across the TR wavefrontemitted by the object being imaged with the DAISI system such as todivide such wavefront spatially in a non-overlapping fashion. To thisend, FIG. 2 illustrates a related embodiment 200, in which a frontoptical portion contributing to the spatial division of the aperture ofthe system is defined by a multiplicity of objective lenses 224configured as a two-dimensional (2D) array of lenses. FIG. 2 presents ageneral view of the system 200 and, in figure insert, a portion 202 ofit in greater detail, including a field reference (aperture stop) 204.The configuration 200 has an operational advantage over embodiment 100of FIG. 1 in that the overall size and/or weight and/or cost ofmanufacture of the embodiment 200 is critically reduced while theassociated parallax (the change in the FOVs of individual lenses 224 ofthe lens-may disposed across and substantially perpendicularly to ageneral optical axis 226 of the embodiment 200; marked as 228) issubstantially small. As the distance between the portion 202 and theobject 110 increases, the overlapping region 230 between the FOVs of theindividual lenses 224 increases while the amount of parallax 228 remainsapproximately the same, thereby reducing its effect on the system 200.When the ratio of the parallax-to-object-distance is substantially equalto the pixel-size-to-system-focal-length ratio then the parallax effectmay be considered to be negligible and, for practical purposes, nolonger distinguishable. While the lenses 224 are shown to be disposedsubstantially in the same plane, optionally the array of front objectivelenses such as lenses 224 can be defined in more than one plane. Forexample, some of the individual lenses 224 can be displaced with respectto some other individual lenses 224 along the axis 226 (not shown). Itis noted that when multiple detectors 236 are employed with theembodiment 200, the embodiment is preferably complemented with fieldreference 204 to operate properly, as discussed below.

In one implementation, the front objective lens system such as the arrayof lenses 224 is configured as an array of lenses integrated or moldedin association with a monolithic substrate, thereby reducing the costsand complexity otherwise accompanying the optical adjustment ofindividual lenses within the system. An individual lens 224 canoptionally include a lens with varying magnification. As one example, apair of thin and large diameter Alvarez plates can be used to define atleast a portion of the front objective lens system.

In further reference to FIG. 1, the FPA component configured to receivethe optical data representing spectral signature(s) of the imaged objectcan be configured as a single FPA 136 adapted to acquire more than onesub-image (formed along more than one optical channel 120)simultaneously. Alternatively, the detector component may include a setof optical FPAs at least one of which can be configured to acquired morethan one spectrally distinct sub-image of the imaged object (Forexample, as shown in the embodiment 200 of FIG. 2, an array of opticalFPAs can include FPAs 236 the number of which may correspond to thenumber of the front objective lenses 224). In one implementation of thesystem, an array of optical FPAs includes an array of microbolometers.The use of multiple microbolometers advantageously allows for aninexpensive way to increase the total number of detection elements (i.e.pixels) for recording of the datacube in one snapshot. An array ofmicrobolometers more efficiently utilizes the detector pixels for eachFPA as the number of unused pixels is minimized and/or eliminatedbetween the sub-images that may exist when using a singlemicrobolometer.

FIG. 3A illustrates schematically a related embodiment 300 of theimaging system of the invention, in which the number of the frontobjective lenses 324 a in the lens array 324, the number of re-imaginglenses 128 a in the lens array 128, and the number of FPAs 336 are thesame. So configured, each combination of respectively correspondingfront objective lens 324, re-imaging lens 128 a, and FPA 336 defines anindividual imaging channel associated with acquisition of the IR lighttransmitted from the object 110 through an individual optical filtercomponent 130. A field reference 338 of the system 300 is configured tohave a uniform temperature across its surface and be characterized by apredetermined spectral curve of radiation emanating therefrom. The filedreference 338 is used for dynamically adjusting the data output fromeach FPA 336 after acquisition of light from the object 110 to ensurethat output of each of the FPAs 336 represents correct acquired data,with respect to the other FPAs 336 for analysis, as discussed below inmore detail. In one implementation, when a 4×3 array 340 of opticalcomponents (lenses 324 a, 128 a; detector elements 336), shownschematically in FIG. 3B, is used behind the temperature controlledreference target 160, the field reference 338 is adapted to obscureand/or block a peripheral portion of the bundle of light propagatingfrom the object 110 towards the detector(s) 336. As a result, the fieldreference 338 obscures and/or blocks the border or peripheral portion(s)of the images of the object 110 formed on the FPA elements located alongthe perimeter 346 of the detector system. Generally, two detectorelements will be producing substantially equal values of digital countswhen they are used to observe the same portion of the scene in the samespectral region using the same optical train. If any of these inputparameters (scene to be observed, spectral content of light from thescene, or optical elements delivering light from the scene to the twodetector elements) differ, the counts associated with the detectors willdiffer as well. Accordingly, and as an example, in a case when the twoFPAs 336 (such as those denoted as #6 and #7 in FIG. 3B) remainsubstantially un-obscured by the field reference 338, the outputs fromthese FPAs can be—dynamically adjusted to the output from one of theFPAs located along border (such as, for example, the FPA element #2)that processes spectrally similar light.

FIG. 4 illustrates schematically a portion of another embodiment 400that contains an array 424 of front objective lenses 424 a adapted toreceive light from the object 110 that relay the received light to thearray 128 of re-imaging lenses 128 a through an array 438 of fieldreferences (field stops) 438 a the spectral characteristics of which areknown, and through an array 440 of the relay lenses. The fieldreferences 438 a are disposed at corresponding intermediate image planesdefined, with respect to the object 110, by respectively correspondingfront objective lenses 424 a. (When refractive characteristics of all ofthe front objective lenses 424 a are substantially the same, all of thefield references 438 a are disposed in the same plane). A fieldreference 438 a of the array 438 obscures (casts a shadow on) aperipheral region of a corresponding sub-image formed at the detectorplane 444 through a respectively corresponding spatial imaging channel450 of the system 400 prior to such sub-image being spectrally processedby the processor 150. The array 440 of relay lenses then transmits lightalong each of the imaging channels 450 through different spectralfilters 454 a of the filter array 454, past the two-point calibrationapparatus that includes two temperature controlled shutters 460 a, 460b, and then onto the detector module 456 (a microbolometer array orother IR FPA).

The embodiment 400 commissions several operational advantages. It isconfigured to provide a spectrally known object within every sub-imageand for every snapshot acquisition which can be calibrated against.(Such spectral certainty is expedient when using an array of IR FPAslike microbolometers the detection characteristics of which can changefrom one imaging frame to the next due to, in part, changes in the scenebeing imaged as well as the thermal effects caused by neighboring FPAs.)In addition, the field reference array 438 of the embodiment 400 ispreferably—but not necessarily—disposed within the Rayleigh range (˜thedepth of focus) associated with the front objective lenses 424, therebyremoving unusable blurred pixels due to having the field referenceoutside of this range. Moreover, the embodiment 400 is more compactthen, for example, the configuration 300 of FIG. 3A (which requires theemployed field reference 338 to be separated from the lens array 324 bya distance greater than several (for example, five) focal lengths tominimize blur contributed by the field reference to an image formed at adetector plane.

In another related embodiment (not shown in FIGS. 1, 2, 3A, and 4), themulti-optical FPA unit of the IR imaging system of the inventionadditionally includes an FPA configured to operate in a visible portionof the spectrum. In reference to FIG. 1, for example, an image of thescene of interest formed by such visible-light FPA may be used as abackground to form a composite image by overlapping (whether virtually,with the use of a processor and specifically-designed computer programproduct enabling such data processing, or actually, by a viewer) an IRimage (that is created based on the image data acquired by theindividual FPAs 130) with the visible-light image. The so-formedcomposite image facilitates the identification of the precise spatiallocation of the target species the spectral signatures of which thesystem of the invention is enabled to detect and recognize.

Optical Filters.

It is appreciated that the optical filters, used with an embodiment ofthe system, that define spectrally-distinct IR sub-images of the objectcan employ absorption filters, interference filters, and Fabry-Perotetalon based filters, to name just a few. When interference filters areused, the image acquisition through an individual imaging channeldefined by an individual reimaging lens (such as a lens 128 a of FIGS.1, 2, 3, and 4) may be carried out in a single spectral bandwidth ormultiple spectral bandwidths. Referring again to the embodiments 100,200, 300, 400 of FIGS. 1 through 4, and in further reference to FIG. 3B,examples of a 4-by-3 array of spectral filters 130 is shown in FIGS. 5A,5B, where individual filters 1 through 12 are juxtaposed with asupporting opto-mechanical element (not shown) to define a filter-arrayplane that is oriented, in operation, substantially perpendicularly tothe general optical axis 226 of the embodiment.

The optical filtering configuration of one present embodimentadvantageously differs from a common approach used to measure spectrawith an array of FPAs, where a bandpass filter defining a specifiedspectral band (such as, for example, any of the filters 0 a through 4 athe transmission curves of which are shown in FIG. 6A) is placed infront of the optical FPA (generally, between the optical FPA and theobject). In particular, and in further reference to FIGS. 1, 2 3, and 4when optical detector(s) 136, 236, 336, 456 of an embodiment include(s)microbolometers, the predominant contribution to noise associated withimage acquisition is due detector noise. To compensate and/or reduce thenoise, an embodiment of the invention utilizes spectrally-multiplexedfilters. An example of spectral transmission characteristics ofspectrally-multiplexed filters 0 b through 4 b for use with anembodiment of the invention is depicted in FIG. 6B. (Filters of FIG. 6Bare so-called long-wavelength pass, LP filters. An LP filter generallyattenuates shorter wavelengths and transmits (passes) longer wavelengthsover the active range of the target IR portion of the spectrum. It isappreciated that, in a related embodiment, short-wavelength-passfilters, SP, may also be used. An SP filter generally attenuates longerwavelengths and transmits (passes) shorter wavelengths over the activerange of the target IR portion of the spectrum.)

The related art appears to be silent with respect to an IR imagingsystem, adapted for detection of spectral signatures of chemical speciesthat combines the use of the spectrally-multiplexed filters with asnap-shot image acquisition. The lack of such teaching can probably beexplained by the fact that related imaging systems require the use ofhighly sensitive and, for that reason, expensive cooled FPAs withreduced noise characteristics. Accordingly, the systems of the relatedart are commonly employing bandpass filters instead, to take fulladvantage of spectral sensitivity of the used FPAs. Simply put, the useof spectrally multiplexed filters such as notched, LP, and SP filterswould be counterproductive in a system of the related art, and would atleast reduce an otherwise achievable SNR thereby degrading theperformance of the related art system for the intended purpose. Incontradistinction with the systems of the related art, however, and atleast in part due to the snap-shot/non-scanning mode of operation, anembodiment of the imaging system of the invention is enabled to use lesssensitive microbolometers without compromising the SNR. The use ofmicrobolometers, as detector-noise-limited devices, in turn not onlybenefits from the use of spectrally multiplexed filters, but also doesnot require cooling of the imaging system during normal operation.

Referring again to FIGS. 6A, 6B, each of the filters (0 b . . . 4 b)transmits light in a substantially wider region of spectrum as comparedto those of the filters (0 a . . . 4 a). Accordingly, when thespectrally-multiplexed set of filters (0 b . . . 0 d) is used with anembodiment of the invention, the overall amount of light received by theFPAs (for example, 236, 336) is larger than would be received when usingthe bandpass filters (0 a . . . 4 a). This “added” transmission of lightdefined by the use of the spectrally-multiplexed LP (or SP) filtersfacilitates increase of the signal on the FPAs above the level of thedetector noise. Additionally, by using, in an embodiment of theinvention, filters the spectra of which are wider than those ofconventionally used band-pass filters, the uncooled FPAs of theembodiment experience less heating due radiation incident thereon fromthe imaged scene and from radiation emanation form the FPA in questionitself, due to a reduction in the back-reflected thermal emission(s)coming from the FPA and reflecting off of the filter from the nonband-pass regions. As the transmission region of the multiplexed LP (orSP) filters is wider, such parasitic effects are reduced therebyimproving the overall performance of the FPA unit.

In one implementation, the LP and SP filters can be combined, in aspectrally-multiplexed fashion as described, in order to maximize thespectral extent of the transmission region of the filter system of theembodiment.

The advantage of using spectrally multiplexed filters is appreciatedbased on the following derivation, in which a system of M filters isexamined (although it is understood that in practice an embodiment ofthe invention can employ any number of filters). For illustration, thecase of M=7 is considered. Analysis presented below relates to onespatial location in each of sub-images formed by differing imagingchannels defined by the system. As similar analysis can be performed foreach point at a sub-image, the analysis can be appropriately extended asrequired.

The unknown amount of light within each of the M spectral channels(corresponding to these M filters) is denoted with f₁, f₂, f₃, . . .f_(M), and readings from corresponding detector elements receiving lighttransmitted by each filter is denoted as g₁, g₂, g₃ . . . g_(M), whilemeasurement errors are represented by n₁, n₂, n₃, . . . n_(M). Then, thereadings at the seven FPA pixels each of which is optically filtered bya corresponding bandpass filter of FIG. 6A can be represented by:

g ₁ =f ₁ +n ₁,

g ₂ =f ₂ +n ₂,

g ₃ =f ₃ +n ₃,

g ₄ =f ₄ +n ₄,

g ₅ =f ₅ +n ₅,

g ₆ =f ₆ +n ₆,

g ₇ =f ₇ +n ₇,

These readings (pixel measurements) g_(i) are estimates of the spectralintensities f_(i). The estimates g_(i) are not equal to thecorresponding f_(i) values because of the measurement errors n_(i).However, if the measurement noise distribution has zero mean, then theensemble mean of each individual measurement can be considered to beequal to the true value, i.e. (g_(i))=f_(i). Here, the angle bracketsindicate the operation of calculating the ensemble mean of a stochasticvariable. The variance of the measurement can, therefore, be representedas:

((g _(i) −f _(i))²)=(n _(i) ²)=g ²

In an alternative design utilizing spectrally-multiplexed filters and incomparison with the design utilizing bandpass filters, the amount ofradiant energy transmitted by each of the spectrally-multiplexed LP orSP filters towards a given detector element can exceed that transmittedthrough a spectral band of a bandpass filter. IN this case, theintensities of light corresponding to the independent spectral bands canbe reconstructed by computational means. (Such design is referred to asa “multiplex design”).

One matrix of such “multiplexed filter” measurements includes a Hadamardmatrix (requiring “negative” filters that may not be necessarilyappropriate for the optical embodiments disclosed herein) An S-matrixapproach (which is restricted to having a number of filters equal to aninteger that is multiple of four minus one) or a row-doubled Hadamardmatrix (requiring a number of filters to be equal to an integer multipleof eight) present alternative methodologies. Here, possible numbers offilters using an S-matrix setup are 3, 7, 11, etc and, if a row-doubledHadamard matrix setup is used, then 8, 16, 24, etc. For example, thegoal of the measurement may be to measure seven spectral band f₁intensities using seven measurements g₁ as follows:

g ₁ =f ₁+0+f ₂+0+f ₅+0+f ₇ +n ₁,

g ₂=0|f ₂ |f ₂|0|0|f ₆ |f ₇ |n ₂

g ₃ =f ₁ +f ₂+0+0+f ₅+0+f ₇ +n ₃

g ₄=0+0+0+f ₄ +f ₃ +f ₇ +f ₈ +n ₄

g ₅ =f ₁+0+f ₃ +f ₄+0+f ₆+0+n ₅

g ₆=0+f ₂ +f ₃ +f ₄ +f ₅+0+0+n ₆

g ₇ =f ₁ +f ₂+0+f ₄+0+0+f ₇ +n ₇

Optical transmission characteristics of the filters described above aredepicted in FIG. 6B. Here, we no longer have a direct estimate of thef_(i) through a relationship similar to (g_(i))=f_(i). Instead, if a“hat” notation is used to denote an estimate of a given value, then alinear combination of the measurements can be used such as, for example,

f ₁=¼(+g ₁ −g ₂ +g ₃ −g ₄ +g ₅ −g ₆ +g ₇),

f ₂=¼(−g ₁ +g ₂ +g ₃ −g ₄ −g ₅ +g ₆ +g ₇)

f ₃=¼(+g ₁ +g ₂ −g ₃ −g ₄ +g ₅ +g ₆ −g ₇),

f ₄=¼(−g ₁ −g ₂ −g ₃ +g ₄ +g ₅ +g ₆ +g ₇),

f ₅=¼(+g ₁ −g ₂ +g ₃ +g ₄ −g ₅ +g ₆ −g ₇),

f ₆=¼(−g ₁ +g ₂ +g ₃ +g ₄ +g ₅ −g ₆ −g ₇),

f ₇=¼(+g ₁ +g ₂ −g ₃ +g ₄ −g ₅ −g ₆ +g ₇),

These {circumflex over (f)}_(i) are unbiased estimates when the n_(i)are zero mean stochastic variables, so that ({circumflex over(f)}_(i)−f_(i))=0. The measurement variance corresponding to ithmeasurement is

${\langle\left( {f_{i} - f_{i}} \right)^{2}\rangle} = {\frac{7}{16}\sigma^{2}}$

Therefore, by employing spectrally-multiplexed system thesignal-to-noise ratio (SNR) of a measurement has been improved by afactor of √{square root over (16/17)}=1.51.

For N channels, the SNR improvement achieved with aspectrally-multiplexed system can be expressed as (N+1)/(2√{square rootover (N)}). For example, in an embodiment employing 12 spectral channelsis characterized by SNR improvement, over a non-spectrally-multiplexedsystem, by a factor of up to 1.88.

Two additional examples of related spectrally-multiplexed filterarrangements 0 c through 4 c, 0 d through 4 d from the use of which anembodiment of the invention can benefit when such embodiment includes anuncooled FPA (such as a microbolometer) are shown in FIGS. 6C and 6D.FIG. 6C illustrates a set of spectrally-multiplexed long-wavelength pass(LP) filters is used in the system. An LP filter generally attenuatesshorter wavelengths and transmits (passes) longer wavelengths over theactive range of the target IR portion of the spectrum. A single spectralchannel having a transmission characteristic corresponding to thedifference between the spectral transmission curved of at least two ofthese LP filters can be used to procure imaging data for the datacubewith an embodiment of the invention.

As alluded to above, an embodiment may optionally, and in addition totemperature-controlled reference unit (for example temperaturecontrolled shutters such as shutters 160; 160 a, 160 b), employ a fieldreference component (338 in FIG. 3A), or an array of field referencecomponents (438 in FIG. 4), to enable dynamic calibration for spectralacquisition of every datacube, a spectrally-neutral camera-to-cameracombination to enable dynamic compensation of parallax artifacts, and avisible and/or IR camera for compensation of motion artifacts. The useof the temperature-controlled reference unit (for example,temperature-controlled shutter system 160) and field-referencecomponent(s) facilitates maintenance of proper calibration of each ofthe FPAs individually and the entire FPA unit as a whole.

In particular, and in further reference to FIGS. 1, 2, 3, and 4, thetemperature-controlled unit generally employs a system having first andsecond temperature zones maintained at first and second differenttemperatures. For example, shutter system of each of the embodiments100, 200, 300 and 400 can employ not one but at least twotemperature-controlled shutters that are substantially parallel to oneanother and transverse to the general optical axis 226 of theembodiment(s) 100, 200, 300, 400. Referring, for example, to FIG. 4, inwhich such multi-shutter structure is already indicated, the use ofmultiple shutters enables the user to create a known referencetemperature difference perceived, by the FPAs 456 through the IRradiation emitted by the shutter(s) 160 a, 160 b when these shutters arepositioned to block the radiation from the object 110. As a result, notonly the offset values corresponding to each of the individual FPAspixels can be adjusted but also the gain values of these FPAs. In analternative embodiment, the system having first and second temperaturezones may include a single or multi-portion piece (such as a plate, forexample) mechanically-movable across the optical axis with the use ofappropriate guides and having a first portion at a first temperature anda second portion at a second temperature.

Indeed, the process of calibration of an embodiment of the inventionstarts with estimating gain and offset (that vary from detector pixel todetector pixel) by performing measurements of radiation emanating,independently, from at least two temperature-controlled shutters ofknown and different radiances. Specifically, first the response of thedetector unit 456 to radiation emanating from one shutter (for example,shutter 160 a that is blocking the FOV of the detectors 456 and thetemperature T₁ of which is measured directly and independently withthermistors) is carried out. Following such initial measurement, theshutter 160 a is removed from the optical path of light traversing theembodiment and another shutter (for example, 160 b) is inserted in itsplace across the optical axis 226 to prevent the propagation of lightthrough the system. The temperature of the second shutter 160 b is T₂≠T₁is also independently measured with thermistors placed in contact withthis shutter, and the detector response to radiation emanating from theshutter 160 b is also recorded. Denoting operational response of FPApixels (expressed in digital numbers, or “counts”) as g_(i) to a sourceof radiance L_(i), the readings corresponding to the measurements of thetwo shutters can be expressed as:

g ₁ =γL ₁(T ₁)+_(offset)

g ₂ =γL ₂(T ₂)+g _(offset)

Here, g_(offset) is the pixel offset value (in units of counts), and γis the pixel gain value (in units of counts per radiance unit). Thesolutions of these two equations with respect to the two unknownsg_(offset) and γ can be obtained if the values of g₁ and g₂ and theradiance values L₁ and L₂ are available (either measured by a referenceinstrument or calculated from the known temperatures T₁ and T₂ togetherwith the known spectral response of the optical system and FPA). For anysubsequent measurement, one can then invert the equation(s) above inorder to estimate the radiance value of the object from the detectormeasurement, and this can be done for each pixel in each FPA arraywithin the system.

As already discussed, and in reference to FIGS. 1 through 4 thefield-reference apertures may be disposed in an object space or imagespace of the optical system, and dimensioned to block a particularportion of the IR radiation received from the object that, in absence ofthe field-reference aperture, would transmit through the optical systemwithout traversing at least two spectrally-multiplexed optical filters.For example, the field-reference aperture the opening of which issubstantially similar, in shape, to the boundary of the filter array(for example, and in reference to a filter array of FIGS. 3B,5B—rectangular), can be placed in front of the objective lens (124, 224,324, 424) at a distance that is at least several times (in oneimplementation—at least five times) larger than the focal length of thelens in order to minimize the image blur that would occur in absence ofsuch field-reference. In the embodiment 400 of FIG. 4 thefield-reference aperture can be placed within the depth of focus of animage conjugate plane formed by the front objective lens 424. The fieldreference, generally, effectuates and/or enables dynamic compensation inthe system by providing a spectrally known and temporally-stable objectwithin every scene to reference and stabilize the output from thedifferent FPAs in the array.

Because each FPA's offset value is generally adjusted from each frame tothe next frame by the hardware, comparing the outputs of one FPA withanother can have an error that is not compensated for by the calibrationparameters g_(offset) and γ. In order to ensure that FPAs operate inradiometric agreement, it is necessary for a portion of each detectorarray to view a reference source (such as the field reference 338 inFIG. 3A, for example). Since the reference source spectrum is known apriori (such as a blackbody source at a known temperature), one canmeasure the response of each FPA to the reference source in order toestimate changes to the pixel offset value. An example calculation ofthe dynamic offset proceeds as follows.

Among the FPA elements in an array of FPAs in a given embodiment of theinvention, we select one FPA to be the “reference FPA”. We will attemptto make all of the FPAs agree with this one about the field referencetemperature. The image measured in each FPA contains a set of pixelsobscured by the field reference 338. Using the previously obtainedcalibration parameters g_(offset) and γ (the pixel offset and gain), weestimate the effective blackbody temperature T of the field reference asmeasured by each FPA i. That is,

T _(i)=mean{(g+Δg _(i) +g _(offset)/γ}=mean{g−g _(offset))/γ}+ΔT _(i)

Here, the mean value is procured over all pixels that are obscured bythe field reference, and Δg_(i) is the difference in offset value of thecurrent frame from Δg_(offset) obtained during the calibration step. Forthe reference FPA, Δg_(i) is simply set to zero. Then, using thetemperature differences measured by each FPA, one obtains

T _(i) −T _(ref)=mean{(g+Δg _(i) +g _(offset) /γ}+ΔT _(i)−mean{(g−g_(offset))/γ}=ΔT _(i)

Once Δt_(i) for each FPA is measured, its value can be subtracted fromeach image in order to force operational agreement between such FPA andthe reference FPA. While the calibration procedure has been discussedabove in reference to calibration of temperature, a procedurally similarmethodology of calibration with respect to radiance value can beimplemented.

Examples of Methodology of Measurements.

Prior to optical data acquisition with an embodiment of the IR imagingsystem of the invention, it is preferred to calibrate all the FPAs ofthe system (such as FPAs 336 each of which forms an image of the objectin light delivered in a corresponding optical channel defined by thecombination of the corresponding front objective and re-imaging lenses324, 128 a, in reference to FIG. 3). The calibration is necessitated bya need to form individual images in equivalent units (so that, forexample, the reading from each of the FPA pixels can be re-calculated inunits of temperature or radiance units). Moreover, while it is oftenneglected in practice, each of the FPAs should be spatiallyco-registered with one another so that a given pixel of a particular FPAcan be confidently optically re-mapped through the optical system of theembodiment to the same location at the object as the corresponding pixelat another FPA.

To achieve at least some of these goals, a so-called spectraldifferencing method may be employed, which employs forming a differenceimage from various combinations of the images registered by two or moredifferent FPAs. If the optical filter 130 corresponding to a particularFPA 336 transmits light from the object including a cloud of gas, forexample, with a certain spectrum that contains the gas absorption peakor a gas emission peak while another filter 130 corresponding to anotherFPA 336 does not transmit such spectrum, then the difference between theimages formed by the two FPAs at issue will highlight the presence ofgas in the difference image.

A shortcoming of the spectral differencing method is that contributionsof some auxiliary features associated with imaging (not just the targetspecies such as gas itself) can also be highlighted in and contribute tothe difference image. The so contributing effects include, to name justa few, parallax-induced imaging of edges of the object, influence ofmagnification differences between the two or more optical channels, anddifferences in rotational positioning and orientation between the FPAs.While magnification-related errors and FPA-rotation-caused errors can becompensated for by increasing the accuracy of the instrumentconstruction as well as by post-processing of the acquired imaging,parallax is scene-induced and is not so easily correctable. In addition,while it is not widely recognized, the spectral differencing method isvulnerable to radiance calibration errors. Specifically, if one FPAregisters radiance of light from a given feature of the object as thathaving a temperature of 40° C., for example, while the data from anotherFPA represents the temperature of the same object feature as being 39°C., then such feature of the object will be enhanced or highlighted inthe difference image (formed at least in part based on the imagesprovided by these two FPAs) due to such radiance-calibration error.

One solution to some of the problems introduced by the spectraldifferencing is to normalize the resulting image data by the datacorresponding to a temporal reference image. This is referred to, forthe purposes of this disclosure, as a temporal differencing algorithm ormethod. A temporal reference image may be formed, for example, bycreating a difference image from the two or more images registered bythe two or more FPAs at a single instance in time. It does not matterwhether corollary of the use of the algorithm of the invention is that aprior knowledge of whether the object or scene contains a target species(such as gas of interest) does not affect the results because thealgorithm highlights changes in the scene characteristics. Then, aspectral difference image can be calculated as discussed above based ona snap-shot image acquisition at any later time and subtracted from thetemporal reference image to form a normalized difference image. Thedifference between the two highlights the target species (gas) withinthe normalized difference image, since this species was not present inthe temporal reference frame. If necessary, more than two FPAs can beused both for registering the temporal reference image and alater-acquired difference image to obtain a better SNR figure of merit.

While the temporal differencing method can be used to reduce oreliminate some of the shortcomings of the spectral differencing, it canintroduce unwanted problems of its own. For example, temporaldifferencing of imaging data is less sensitive to calibration andparallax induced errors than the spectral differencing of imaging data.However, any change in the imaged scene which is not related to thetarget species of interest (such as particular gas, for example) ishighlighted in a temporally-differenced image, and thus may beerroneously perceived as a location of the target species triggering,therefore, an error in detection of target species. For example, if thetemperature of the scenic background against which the gas is beingdetected changes (due to natural cooling down as the day progresses, orincreases due to a person or animal or another object passing throughthe FOV of the IR imaging system), then such temperature change producesa signal difference as compared to the measurement taken earlier intime. Accordingly, the cause of the scenic temperature change (thecooling object, the person walking, etc.) may appear as the detectedtarget species (such as gas). It follows, therefore, that an attempt tocompensate for operational differences among the individual FPAs of amulti-FPA IR imaging system with the use of methods that turn onspectral or temporal differencing cause additional problems leading tofalse detection of target species. Among these problems arescene-motion-induced detection errors and parallax-caused errors thatare not readily correctable and/or compensatable as of to-date.Accordingly, there is an unfulfilled need to compensate for image dataacquisition and processing errors caused by motion of elements withinthe scene being imaged. Embodiments of data processing algorithms of thepresent invention address and fulfill the need to compensate for themotion-induced and parallax-induced image detection errors.

In particular, to minimize parallax-induced differences between theimages produced with two or more predetermined FPAs, another differenceimage is used that is formed from the images of at least two differentFPAs to estimate parallax effects. For example, the spectraldifferencing of the image data is being performed with the use of thedifference between the images collected by the outermost two cameras inthe array (such as, for example, the FPAs corresponding to filters 2 and3 of the array of filters of FIG. 5A), forming a difference imagereferred to as a “difference image 2-3”. In this case, the alternative“difference image 1-4” is additionally formed from the image dataacquired by, for example, the alternative FPAs corresponding to filters1 and 4 of FIG. 5A. Assuming or ensuring that both of these twoalternative FPAs have approximately the same spectral sensitivity to thetarget species, the alternative “difference image 1-4” will highlightpixels corresponding to parallax-induced features in the image.Accordingly, based on positive determination that the same pixels arehighlighted in the spectral “difference image 2-3” used for targetspecies detection, the conclusion is made that the image featurescorresponding to these pixels are likely to be induced by parallax andnot the presence of target species in the imaged scene. It should benoted that compensation of parallax can also be performed using imagescreated by individual re-imaging lenses, 128 a, when using a single FPAor multiple FPA's as discussed above.

Another capability of the embodiment of the invention is the ability toperform the volumetric estimation of a gas cloud volumetric estimation.This can be accomplished by using (instead of compensating or negating)the parallax-induced effects described above. In this case, the measuredparallax between two or more similar spectral response images can beused to estimate a distance between the imaging system and the gas cloudor between the imaging system and an object in the field of view of thesystem. The parallax-induced transverse image shift d between two imagesis related to the distance z between the cloud or object and the imagingsystem according to z=−sz′/d, where s is the separation between twosimilar spectral response images, and z′ is the distance to the imageplane from the back lens (z′ is typically approximately equal to thefocal length f of the lens of the imaging system). Once the distance zbetween the cloud and the imaging system is calculated, the size of thegas cloud can be determined based on the magnification equation, m=f/z,where each image pixel on the gas cloud, Δx′, corresponds to a physicalsize in object space Δx=Δx′/m. To estimate the volume of the gas cloud,a particular symmetry in the thickness of the cloud based on thephysical size of the cloud can be assumed. For example, the cloud imagecan be rotated about a central axis running through the cloud image tocreate a three dimensional volume estimate of the gas cloud size. It isworth noting that only a single imaging system of the invention isrequired for such volume estimation, in contradistinction with carryingout such estimate with a spectral imaging system of related art (inwhich case at least two imaging systems would be necessary). Indeed, dueto the fact that the information about the angle at which the gas cloudis seen by the system is decoded in the parallax-effect, the image dataincludes the information about the imaged scene viewed by the system inassociation with at least two angles.

When the temporal differencing algorithm is used for processing theacquired imaging data, a change in the scene that is caused not by thetarget species is highlighted in the resulting image. According to anembodiment of the invention, compensation of this error makes use of thetemporal differencing between two FPAs that are substantially equallyspectrally sensitive to the target species. In this case, the temporaldifference image will highlight those pixels the spectra of which havechanged in time. Subtracting the data corresponding to these pixels atboth FPAs to form the resulting image, therefore, excludes thecontribution of the target species to the resulting image. Thedifferentiation between (i) changes in the scene due to the presence oftarget species and (ii) changes in the scene caused by changes in thebackground not associated with the target species is, therefore,enabled. It should be noted that, quite unexpectedly, the data acquiredwith the visible-light FPA (when present as part of the otherwise IRimaging system) can also be used to facilitate such differentiation andcompensation of the motion-caused imaging errors. Visible camerasgenerally have much lower noise figure than IR cameras (at least duringdaytime). Consequently, the temporal difference image obtained with theuse of image data from the visible-light FPA can be quite accurate. Thevisible FPA can be used to compensate for motion in the system as wellas many potential false-alarms in the scene due to motion caused bypeople, vehicles, birds, and steam, for example, as long as the movingobject can be observed in the visible region of the spectra. This hasthe added benefit of providing an additional level of false alarmsuppression without reducing the sensitivity of the system since manytargets such as gas clouds cannot be observed in the visible spectralregion.

Another method for detection of the gases is to use a spectral unmixingapproach. A spectral unmixing approach assumes that the spectrummeasured at a detector pixel is composed of a sum of component spectra,and attempts to estimate the relative weights of these components neededto derive the measurement spectrum. The component spectra are generallytaken from a predetermined spectral library (for example, from datacollection that has been empirically assembled), though sometimes onecan use the scene to estimate these as well (often called “endmemberdetermination”). For the gas cloud detection, the component spectrainclude the absorption spectra of various gases of interest, while the“measurement spectrum” is not the raw measurement of spectral intensityvalues but rather an “absorption spectrum”, which includes the spectrumof background light absorbed on transmission through a cloud Thespectral unmixing methodology can also benefit from temporal, parallax,and motion compensation techniques.

Examples of Practical Embodiments and Operation.

The embodiment 300 of FIG. 3 was configured to employ 12 opticalchannels and 12 corresponding microbolometer FPAs as 336 and used tocapture a video sequence (representing images of a standard laboratoryscene) substantially immediately after performing calibrationmeasurements with the use of a reference source including two shutters,as discussed above (one at room temperature and one 5° C. above roomtemperature). The use of 12 FPAs offers a good chance of simultaneousdetection and estimation of the concentrations of about 8 or 9 gasespresent at the scene, but the number of FPAs 336 can vary, depending onthe balance between the operational requirements and consideration ofcost.

Due to the specifics of operation in the IR range of the spectrum, theuse of the so-called noise-equivalent temperature difference (or NETD)is preferred and is analogous to the SNR commonly used in visiblespectrum instruments. The array of microbolometer FPAs 336 ischaracterized to perform at NETD≦72 mK at an f-number of 1.2. Eachmeasurement was carried out by summing four consecutive frames, and thereduction in the NETD value expected due to such summation would bedescribed by corresponding factor of √4=2. Under ideal measurementconditions, therefore, the FPA NETD should be about 36 mK.

The commercially off-the-shelf LP, SP, and/or BP filters were used asthe filters 130. Using the image data acquired at each FPA pixel, theprocessor of the system was used to calculate the mean value and thestandard deviation of the image data across the video sequence, to forma data output representing a “mean image” and a “standard deviationimage” of the scene.

Table 1 summarizes the “mean NETD” values obtained by averaging of theNETD values over all pixels of the standard deviation image”, as well asthe median NETD values obtained in a similar fashion, in degreesCelsius. The top section of Table 1 shows the results for the bandpassfilters, the middle section of Table 1 shows the results for the LP andSP filters, and the bottom section of Table 1 presents data fordifferences between NETD values of two specified LP filters. (Note thatno filter is treated as equivalent to a longpass filter here.)

The results of Table 1 indicate that the difference between the NETDvalues of the two chosen LP filters is substantially smaller than theNETD value corresponding to a single BP filter, thereby providing anexperimental proof that the use of an embodiment of the invention asdiscussed above provides an unexpected increase in a SNR-type figure ofmerit of the IR spectral imaging in comparison with a system of relatedart. In other words, the use of two (or more) LP or SP filters toextract the imaging data results in a spectrally-narrowed imagingchannel having a higher operational performance as compared with the useof a bandpass filter centered on the chose wavelength in the samespectral channel.

It is worth noting that the use of optically-filtered FPAs rather than amore conventional Fourier Transform spectrometer (FTS) in an embodimentof the invention is partly explained by a larger number of total pixelsavailable with a single large format microbolometer FPA array. Moreimportantly, however, the use of the FTS is well recognized to requiretight mechanical tolerances, leading to sufficiently more complexassembly of the imaging system employing the FTS as compared to theassembly of the embodiment of the invention. Additionally, the FTS doesnot offer a high enough optical throughput for a substantially lownumber of optical channels (for example, between 4 and 50 opticalchannels) (in part because many of the sampled wavenumber values in thereconstructed spectrum do not correspond to regions of the spectrum thatthe FTS instrument is sensitive to, and so such sampled data iseventually discarded and not used for image formation and detection oftarget species). The FTS is better suited to higher resolutionspectroscopy. The problem with working with highly-resolved spectra,however, is that by sampling the same amount of incident light withsmaller spectral bins means that image data corresponding each bin isactually noisier. Therefore, while improved spectral resolution accordedby the FTS can allow the user to pick locations in the spectrum that arehighly specific to the absorption/emission signature of the targetspecies, it also makes such signature weaker relative to the detectionnoise.

A major advantage of the embodiments of the present system overinstruments of the related art that are configured for target speciesdetection (for example, gas cloud detection) is that, according to thepresent invention, the entire spectrum is resolved in a snapshot mode(for example, during one image frame acquisition by the FPA array). Thisenables the system of the invention to take advantage of thecompensation algorithms such as the parallax and motion compensationalgorithms mentioned above. Indeed, as the imaging data required toimplement these algorithms are collected simultaneously with thetarget-species related data, the compensation algorithms are carried outwith respect to target-species related data and not with respect to dataacquired at another time interval, thereby ensuring accuracy of the datacompensation process. In addition, the frame rate of data acquisition ismuch higher (the present system operates at up to video rates; fromabout 5 Hz and higher, for example), so that the user is enabled torecognize in the images the wisps and swirls typical of gas mixingwithout blurring out of these dynamic image features and other artifactscaused by the change of scene (whether spatial or spectral) during thelengthy measurements. In contradistinction with the imaging systems ofthe related art that require image data acquisition over a period oftime exceeding a single-snap-shot time and, therefore, blur the targetgas features in the image and inevitably reduce the otherwise achievablesensitivity of the detection, embodiments of the present invention makedetecting the localized concentrations of gas without it being smearedout and/or averaged with the areas of thinner gas concentrations. Inaddition, the higher frame rate also enables a much faster response rateto a leak of gas (when detecting such leak is the goal): an alarm cantrigger within fractions of a second rather than several seconds.

TABLE 1 NETD (deg C.) filter mean median BP-8224 0.691 0.675 BP-90000.938 0.923 BP-9480 0.318 0.315 BP-9740 0.372 0.369 BP-10240 0.275 0.286BP-10700 0.409 0.405 BP-10962 0.44 0.437 BP-11660 0.548 0.542 BP-122270.48 0.475 BP-13289 1.309 1.26 [none] 0.038 0.037 LP-8110 0.063 0.063LP-8500 0.076 0.075 LP-8110 0.068 0.067 LP-8305 0.068 0.067 LP-8500 0.080.08 LP-9000 0.073 0.073 LP-9650 0.099 0.098 LP-9800 0.109 0.108LP-11000 0.156 0.156 LP-11450 0.207 0.206 SP-10500 0.07 0.07 [none] −[LP-8110] 0.07 (i.e. methane) [LP-9650] − [LP-11450] 0.208 (i.e.propylene) [LP-9800] − [LP-11000] 0.227 (i.e. butane)

To demonstrate the operation and gas detection capability of anembodiment of the invention, a prototype was constructed in accordancewith the embodiment 300 of FIG. 3A and used to detect a hydrocarbon gascloud of propylene at a distance of approximately 10 feet. FIG. 7illustrates video frames 1 through 12 representing gas-cloud-detectionoutput 710 (seen as a streak of light) in a sequence from t=1 to t=12.The images 1 through 12 are selected frames taken from a video-datasequence captured at a video-rate of 15 frames/sec. The detectedpropylene gas is shown as a streak of light 710 (highlighted in red)near the center of each image. The first image is taken just prior tothe gas emerging from the nozzle of a gas-contained, while the lastimage represents the system output shortly after the nozzle has beenturned off.

Using the same prototype of the system, the demonstration of the dynamiccalibration improvement described above by imaging the scene surroundingthe system (the laboratory) with known temperature differences. Theresult of implementing the dynamic correction procedure is shown inFIGS. 8A, 8B, where the curves labeled “obj” (or “A”) representtemperature estimates of an identified region in the scene. The abscissain each of the plots of FIGS. 8A, 8B indicates the number of a detectorelement, while the ordinate corresponds to temperature (in degrees C.).Accordingly, it is expected that when all detector elements receiveradiant data that, when interpreted as the object's temperature,indicates that the object's temperature perceived by all detectorelements is the same, any given curve would be a substantially flatline. Data corresponding to each of the multiple “obj” curves are takenfrom a stream of video frames separated from one another by about 0.5seconds (for a total of 50 frames). The recorded “obj” curves shown inFIG. 8A indicate that the detector elements disagree about the object'stemperature, and that difference in object's temperature perceived bydifferent detector elements is as high as about 2.5° C. In addition, allof the temperature estimates are steadily drifting in time, from frameto frame. The curves labelled “ref” (or “C”) correspond to thedetectors' estimates of the temperature of the aperture 338 of theembodiment 300 of FIG. 3A. The results of detection of radiation carriedout after each detector pixel has been subjected to the dynamiccalibration procedure described above are expressed with the curvedlabeled “obj corr” (or “B”). Now, the difference in estimatedtemperature of the object among the detector elements is reduced toabout 0.5° C. (thereby improving the original reading at least by afactor of 5).

FIG. 8B represents the results of similar measurements corresponding toa different location in the scene (a location which is at a temperatureabout 9° C. above the estimated temperature of the aperture 338 of FIG.3A). As shown, the correction algorithm discussed above is operable andeffective and applicable to objects kept at different temperature.Accordingly, the algorithm is substantially temperature independent.

At least some elements of a device of the invention can becontrolled—and at least some steps of a method of the invention can beeffectuated, in operation—with a programmable processor governed byinstructions stored in a memory. The memory may be random access memory(RAM), read-only memory (ROM), flash memory or any other memory, orcombination thereof, suitable for storing control software or otherinstructions and data. Those skilled in the art should also readilyappreciate that instructions or programs defining the functions of thepresent invention may be delivered to a processor in many forms,including, but not limited to, information permanently stored onnon-writable storage media (e.g. read-only memory devices within acomputer, such as ROM, or devices readable by a computer I/O attachment,such as CD-ROM or DVD disks), information alterably stored on writablestorage media (e.g. floppy disks, removable flash memory and harddrives) or information conveyed to a computer through communicationmedia, including wired or wireless computer networks. In addition, whilethe invention may be embodied in software, the functions necessary toimplement the invention may optionally or alternatively be embodied inpart or in whole using firmware and/or hardware components, such ascombinatorial logic, Application Specific integrated Circuits (ASICs),Field-Programmable Gate Arrays (FPGAs) or other hardware or somecombination of hardware, software and/or firmware components.

While examples of embodiments of the system and method of the inventionhave been discussed in reference to the gas-cloud detection, monitoring,and quantification (including but not limited to greenhouse gases suchas Carbon Dioxide, Carbon Monoxide, Nitrogen Oxide as well ashydrocarbon gases such as Methane, Ethane, Propane, n-Butane,iso-Butane, n-Pentane, iso-Pentane, neo-Pentane, Hydrogen Sulfide,Sulfur Hexafluoride, Ammonia, Benzene, p- and m-Xylene, Vinyl chloride,Toluene, Propylene oxide, Propylene, Methanol, Hydrazine, Ethanol,1,2-dichloroethane, 1,1-dichloroethane, Dichlorobenzene, Chlorobenzene,to name just a few), embodiments of the invention can be readily adaptedfor other chemical detection applications. For example, detection ofliquid and solid chemical spills, biological weapons, tracking targetsbased on their chemical composition, identification of satellites andspace debris, ophthalmological imaging, microscopy and cellular imaging,endoscopy, mold detection, fire and flame detection, and pesticidedetection are within the scope of the invention.

1. (canceled)
 2. (canceled)
 3. An infrared (IR) imaging system, theimaging system comprising: an optical system including an optical focalplane array (FPA) unit, the optical system including componentsassociated with two optical channels, said two optical channels beingspatially and spectrally different from one another, each of the twooptical channels positioned to transfer IR radiation incident on theoptical system towards the optical FPA unit, the optical FPA unitcomprising two groups of detector pixels disposed at a distance from twocorresponding focusing lenses; a thermal reference, wherein radiationemitted from the thermal reference directed towards the optical FPA unitis received by at least one of the two groups of detector pixelssimultaneously with radiation emitted from a scene viewed by the imagingsystem; and a data-processing system, said data-processing systemconfigured to: acquire a plurality of frames with the two groups ofdetector pixels having regions in the plurality of image frames thatcorrespond to light of the thermal reference; and dynamically calibratethe two groups of detector pixels based on a measurements obtained bythe two groups of detector pixels using the light received from thethermal reference.
 4. The system of claim 3, wherein the optical FPAunit does not require a cooler to cool during normal operation.
 5. Thesystem of claim 3, wherein the IR radiation comprises wavelengthsbetween about 1 micron and about 20 microns.
 6. The system of claim 3,wherein the processor is configured to map an overall image datacuberepresenting a spatial distribution of concentrations c of the targetspecies.
 7. The system of claim 3, wherein the system is configured toidentify target species in the scene.
 8. The system of claim 3, furthercomprising two spectral filters, each filter positioned in a respectiveone of said two optical channels.
 9. The system of claim 8, wherein atleast one of the two spectral filters is a long pass (LP) filter. 10.The system of claim 8, wherein at least one of the two spectral filtersis a short pass (SP) filter.
 11. The system of claim 8, furthercomprising at least two reimaging lenses, each reimaging lens positionedto transmit a portion of the IR radiation transmitted through arespective one of said two spectral filters towards the optical FPAunit.
 12. The system of claim 3, wherein the thermal reference is afield reference comprising an optical aperture that circumscribes saidtwo optical channels.
 13. The system of claim 3, wherein the thermalreference has a uniform temperature across its surface.
 14. The systemof claim 3, wherein the thermal is configured to obscure a peripheralportion of the incident IR radiation.
 15. The system of claim 3, whereina parameter of one of the two groups of detector pixels is adjustedbased on the measurement obtained by the other of the two groups ofdetector pixels
 16. The system of claim 15, wherein a parameter of oneof the two groups of detector pixels is adjusted based on themeasurement obtained by the other of the two groups of detector pixels.17. The system of claim 3, the measurements obtained by the two groupsof detector pixels using the light received from the thermal referencesare temperature estimates of the thermal references.
 18. The system ofclaim 17, wherein a parameter of one of the two groups of detectorpixels is adjusted based on the temperature estimate of the thermalreference obtained by the other of the two groups of detector pixels.19. The system of claim 3, wherein one of the two groups of detectorpixels comprises a first focal plane array and the other of the twogroups of detector pixels comprises a second focal plane array.